1. Field of the Invention
The present invention relates to multilinear charge transfer array. It relates, more particularly but not exclusively to multilinear charge transfer arrays formed from several elementary arrays and operating in accordance with the principle of integration with charge transfer known as "time delay integration" or TDI.
2. Description of the Prior Art
The multilinear arrays known at present are formed by N lines of P photosensitive detectors such as photodiodes, each line receiving successively the radiation to be detected and by devices for summing, in synchronism with the movement, the information detected on the detectors of the same rank, the whole being integrated on the same substrate. In this case, the integration time is multiplied by N. Now, with the integration time multiplied by N and since the read out noises are added together quadratically, a gain on the signal to noise ratio equal to .sqroot.N is obtained. This advantage is particularly interesting when these arrays are used for sensing radiological images, for the increase of the signal to noise ratio allows the radiation dose used to be reduced for the same exposure time. In the same way, this type of array may be used for detecting other radiations in particular when it is desired to detect low level radiation.
However, the fact of integrating the TDI function directly on the multilinear array has a number of disadvantages.
The integration of the detectors and the devices providing the TDI function requires substrates of relatively large size and is costly to carry out. Furthermore, the scanning direction is imposed. In addition, during operation, problems of thermo-charge generation are met with, whence the necessity of operating the multilinear array at high frequencies and relatively low temperatures.
Moreover, when it is desired to be able to record large sized images several multilinear arrays are frequently associated so as to form a large sized photosensitive array. In this case, however, a loss of resolution and sensitivity is observed at the transition from one array to the next, which losses are due to the absence of one or more photosensitive detectors at the ends of each line of the elementary arrays. A solution to each problem is to dispose the alignment of the disturbed zones situated at the ends of each of the lines in a direction different from the scanning direction. However, it is not possible to provide such scanning when the TDI devices are integrated with the photosensitive device, for, since the cumulative total is made on photosensitive detectors aligned with the scanning direction, it is then necessary to cumulate information situated on two different elementary arrays.